Some good input from others.
As for me, I would consider the 2006 PC2551/E350 if it was in decent shape and priced accordingly. I think the weight distribution on them is better than for the 2350/E350 (what we own) of the same generation which makes up for some of the difference being a little heavier. One good thing about the E350 is that it will offer a softer ride. I have not read so in a very long time, but I do recall long ago, owners complaining that their 2551 on the E450 had a rough ride. It might have had too much surplus of chassis capability over actual weight, feeling more like an empty box truck bouncing down the road. Maybe they resolved the problem by carrying more stuff ???
One thing certain, if the previous 2006/E350 owner did not invest in heavy duty stabilizer bars, I would plan to invest in them for decent handling. The E350 made before 2008 has NO rear stabilizer bar unless added later. And the stock Ford-installed front stabilizer bar is very wimpy, adding that the poorly designed end bushings wear out exceptionally fast rendering the wimpy bar even less effective. Heavy duty front & rear stabilizer bars will do miracles for the handling, and they hold up beautifully for they mount differently. Much of the problem goes away with a 2008 or newer Ford chassis E350 and E450 for the front bar design is better. Even though not heavy duty, because there is one in front and back and mounted well for both E350 & E450, they are more effective than 2007 and older.
I think 2007 was the first year PC came standard with two batteries and the whole house inverter, so keep that in-mind if critical for you. I personally would be okay with a single house battery if all house-battery-supported lighting is converted to low wattage LED to conserve electricity, and don't exclusively camp without power-pole electricity. Two batteries are essential without hook ups in cold weather and relying on the furnace overnight. Remember.......when your house battery(s) is dead, so is your fridge.
I would not worry about E350 part availability because the Ford E-Series is extremely popular. Parts for old E350s are everywhere in stock and will likely remain so for many more years to come for there are so many older Ford E-series vans, box trucks, ambulances, and motor homes abound. The proof is that you can easily get parts anywhere in the USA for the E-series made in the late 80's, and the chassis has been more popular than ever since those days.
As far as PC specific house parts are concerned, most are used today in 2016 model years. I would not worry about that either.
Appliances and such might be another story in later years. You might have to replace an appliance 10 to 15 years from now, instead of repair it. But then maybe it's time to replace anyway for the sake of age alone.
A garage kept (or otherwise protected) 2006 2551 cream puff, priced right would be under my consideration. Not so much if weather-beaten with dried up caulk and showing water damage. Paying $20,000 more to get a rig 5 years newer, updated and well preserved could be worth the extra cost.
The cost percentage makes a little difference to me as well.
If the 2006 is $30,000 and the 2011 is $50,000 that is 70% more. I may favor the 2006
If the 2006 is $50,000 and the 2011 is $70,000 that is 40% more. I may favor the 2011
Ron here with all my bla bla bla.....

Sorry if it's a bit much.